Production scale fabrication method for high resolution afm tips

ABSTRACT

A method of fabricating high resolution atomic force microscopy (AFM) tips including a single semiconductor nanowire grown at an apex of a semiconductor pyramid of each AFM tip is provided. The semiconductor nanowire that is grown has a controllable diameter and a high aspect ratio, without significant tapering from the tip of the semiconductor nanowire to its base. The method includes providing an AFM probe including a semiconductor cantilever having a semiconductor pyramid extending upward from a surface of said semiconductor cantilever. The semiconductor pyramid has an apex. A patterned oxide layer is formed on the AFM probe. The patterned oxide layer has an opening that exposes the apex of the semiconductor pyramid. A single semiconductor nanowire is grown on the exposed apex of the semiconductor pyramid utilizing a non-oxidized Al seed material as a catalyst for nanowire growth.

BACKGROUND

The present disclosure relates to a method of fabricating highresolution Atomic Force Microscopy (AFM) tips and, more particularly toa method of fabricating high resolution AFM tips that have acontrollable diameter and a high aspect ratio.

Atomic force microscopy (AFM) is a very high resolution type of scanningprobe microscopy, with demonstrated resolution on the order of fractionsof a nanometer, which is more than 1000× better than the opticaldiffraction limit. The AFM is one of the foremost tools for imaging,measuring, and manipulating matter at the nanoscale level. Theinformation is gathered by “sensing” the surface with a mechanical probeor tip. Piezoelectric elements that facilitate tiny but accurate andprecise movements on (electronic) command enable very precise scanning.In some variations, using conducting cantilevers is necessary. In newermore advanced AFMs, current can even be passed through the tip to probethe electrical conductivity or transport of the underlying surface.

The AFM consists of a cantilever with a sharp tip at its end that isused to scan a sample surface. The cantilever is typically silicon orsilicon nitride with a tip radius of curvature on the order ofnanometers. When the tip is brought into proximity of a sample surface,forces between the tip and the sample lead to a deflection of thecantilever according to Hooke's law. Depending on the situation, forcesthat are measured in AFM include mechanical contact force, van der Waalsforces, capillary forces, chemical bonding, electrostatic forces,magnetic forces, Casimir forces, solvation forces, etc. Along withforce, additional quantities such as, electrical potential,conductivity, and/or transport, may simultaneously be measured throughthe use of specialized types of probes.

Conventional AFM tips are fabricated by anisotropic etching of silicon,Si. These tips are formed at the end of a silicon cantilever andtypically have a shape of a pyramid with triangular sides defined by Si(111) surfaces. The drawback of such prior art AFM tips is their poorlateral imaging resolution due to their low aspect ratio and largeradius of curvature of about 5 nm-30 nm (best case).

When using AFM to image a property of the surface, the resolution isstrongly dependent on the diameter and aspect ratio of the tip. In someinstances, a high aspect ratio AFM tip is important when imaging steeplyrising steps or narrow trenches. A high aspect ratio is also importantwhen the forced sensed is a long range force. When the force is a longrange force, the force decays as a/r^(n) wherein “a” is a constant, n isa positive integer and r is the distance of the tip to the surface. Insuch a case, not just the atoms at the apex contribute to sensing butthe whole tip contributes to the interaction with the surface. Also, insuch a case, the higher the aspect ratio and the smaller the tip radius,the better the resolution. The current solutions to this problemincluding forming nano-materials such as carbon nanotubes orsemiconductor nanowires at the end of the Si tip have limited aspectratios, are not production scale processes, require more difficultlithographic techniques, or have limited control as to the placement andmorphology of the tip enhancements.

As such, a method is needed that is capable of fabricating highresolution AFM tips that have a controllable diameter as well as a highaspect ratio. Also, there is a need for providing high resolution AFMtips that have little or no tapering associated therewith.

SUMMARY

The present disclosure provides a method of fabricating high resolutionAFM tips including a single semiconductor nanowire grown at an apex of asemiconductor pyramid of each AFM tip which has a controllable diameter(from 5 nm to 200 nm with additional thinning) and a high aspect ratio,e.g., length to width ratio, (on the order of greater than 100), withoutsignificant tapering from the tip of the semiconductor nanowire to itsbase. The term “without significant tapering” is used throughout thepresent application to denote a semiconductor nanowire where the wirediameter changes by less than about 10% from the base of the wire to thetip of the wire. The AFM tips that are provided herein have a highlateral imaging resolution.

In one aspect of the present disclosure, a method of fabricating a highresolution AFM tip having the above mentioned properties is providedthat includes providing an AFM probe including a semiconductorcantilever having a semiconductor pyramid extending upward from asurface of the semiconductor cantilever. The semiconductor pyramidincludes an apex. A patterned oxide layer is formed on the AFM probe.The patterned oxide layer has an opening that exposes the apex of thesemiconductor pyramid. A single semiconductor nanowire is grown on theexposed apex of the semiconductor pyramid utilizing a non-oxidized Alseed material as a catalyst for nanowire growth. Since the singlesemiconductor nanowire is grown from the exposed apex of thesemiconductor pyramid, the semiconductor nanowire is an epitaxialsemiconductor nanowire. The semiconductor nanowire that is formedincludes a p-doped semiconductor nanowire core that is surrounded by anundoped semiconductor shell. The non-oxidized Al seed material ispresent at the tip of the p-doped semiconductor nanowire core. Thenon-oxidized Al seed material and the undoped semiconductor shell can beremoved providing a p-doped semiconductor nanowire extending from theapex of the semiconductor pyramid.

The p-doped semiconductor nanowire that extends from the apex of thesemiconductor pyramid is conductive and has a controllable diameter anda high aspect ratio, without significant tapering from the tip of thep-doped semiconductor nanowire to its base. Also, the p-dopedsemiconductor nanowire that is provided by the present disclosure isstiff enough such that it can be inserted into a biological cell.Moreover, the method of the present disclosure can be used to tailor thedimension and the direction of the p-doped semiconductor nanowire thatis produced according to specific imaging needs. Furthermore, the methodof the present disclosure can also be employed for the mass productionof AFM tips which can be easily implemented into any standard Si AFMcantilever.

The growth of semiconductor nanowires using an Al-based catalyticprocess as disclosed in the present application is different fromsemiconductor nanowires that are grown using a gold (Au) catalyst.First, Al selectively grows a semiconductor nanowire only on a baresemiconductor surface not from an oxide surface. Gold, however, grows asemiconductor nanowire from the oxide surface, in addition to a Sisurface. Further, and as stated above, semiconductor nanowires grownfrom Al provide a unique opportunity to sharpen the AFM tip which cannotbe obtained from semiconductor nanowires grown from Au. Specifically,semiconductor nanowires grown from Al provide a doped semiconductor corewith a radial undoped semiconductor shell surrounding the core. Theundoped semiconductor shell surrounding the core can be selectivelyremoved forming a high resolution AFM tip that has a controllablediameter and a high aspect ratio without significant tapering. Inaddition to the above, Al is easier to remove than Au, and Al is moreelectronically friendly than Au as it does not form deep traps insilicon that could degrade device characteristics.

In another aspect of the present disclosure, a high resolution AFM tipis provided that includes an AFM probe including a semiconductorcantilever having a semiconductor pyramid extending upward from asurface of the semiconductor cantilever, the semiconductor pyramidhaving an apex; and a single Al-doped semiconductor nanowire on theexposed apex of the semiconductor pyramid, wherein the single Al-dopedsemiconductor nanowire is epitaxial with respect to the apex of thesemiconductor pyramid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation (through a cross sectional view)depicting an AFM probe including a semiconductor cantilever having asemiconductor pyramid extending upward from a surface of thesemiconductor cantilever that can be employed in one embodiment of thepresent disclosure.

FIG. 2 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 1 after forming an oxide layer on theAFM probe.

FIG. 3 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 2 after forming a photoresist on anupper surface of the oxide layer.

FIG. 4 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 3 after patterning the photoresist toinclude an opening that exposes the oxide layer that is present on anapex of the semiconductor pyramid.

FIG. 5 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 4 after removing the exposed oxide layerfrom the apex of the semiconductor pyramid exposing the apex of thesemiconductor pyramid.

FIG. 6 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 5 after removing the patternedphotoresist.

FIG. 7 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 6 after forming a layer of aluminum onthe remaining oxide layer and on the exposed apex of the semiconductorpyramid.

FIG. 8 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 7 after annealing.

FIG. 9 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 8 after growing a semiconductor nanowireon the exposed apex of the semiconductor pyramid using a non-oxidized Alseed material that is formed during annealing as a catalyst for nanowiregrowth.

FIG. 10 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 9 after removing an undoped shellportion of the semiconductor nanowire and the remaining oxide layerleaving an aluminum doped semiconductor nanowire on the apex of thesemiconductor pyramid, wherein the aluminum doped semiconductor nanowirehas a high aspect ratio (greater than 100:1) with little or no tapering.

FIGS. 11-12 are pictorial representations (through cross sectionalviews) depicting the structure of FIG. 10 after performing successivestages of an oxidation sharpening process.

FIGS. 13-16 are pictorial representations (through cross sectionalviews) depicting another embodiment of the present disclosure fordefining the region at the apex of the semiconductor pyramid fornanowire growth.

FIGS. 17-21 are pictorial representations (through cross sectionalviews) depicting a further embodiment of the present disclosure fordefining the region at the apex of the semiconductor pyramid fornanowire growth.

DETAILED DESCRIPTION

The present disclosure, which provides a method of fabricating highresolution AFM tips, will now be described in greater detail byreferring to the following discussion and drawings that accompany thepresent disclosure. It is noted that the drawings are provided forillustrative purposes only and are not drawn to scale.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to illustrate the presentdisclosure. However, it will be appreciated by one of ordinary skill inthe art that various embodiments of the present disclosure may bepracticed without these, or with other, specific details. In otherinstances, well-known structures or processing steps have not beendescribed in detail in order to avoid obscuring the various embodimentsof the present disclosure.

It will be understood that when an element as a layer, region orsubstrate is referred to as being “on” or “over” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

As stated above and in one aspect of the present disclosure, a method offabricating a high resolution AFM tip having a controllable diameter anda high aspect ratio, without significant tapering is provided. The AFMtips that are provided herein have a high lateral imaging resolution.The method includes providing an AFM probe including a semiconductorcantilever having a semiconductor pyramid extending upward from asurface of the semiconductor pyramid. The semiconductor pyramid has anapex. A patterned oxide layer is formed on the AFM probe. The patternedoxide layer has an opening that exposes the apex of the semiconductorpyramid. A single semiconductor nanowire is grown on the exposed apex ofthe semiconductor pyramid using a non-oxidized aluminum seed material asa catalyst for nanowire growth. The semiconductor nanowire that isformed includes a p-doped semiconductor nanowire core that is surroundedby an undoped semiconductor shell. The non-oxidized aluminum seedmaterial is present at the tip of the p-doped semiconductor nanowirecore. The non-oxidized aluminum seed material and the undopedsemiconductor shell can be removed providing a p-doped semiconductornanowire extending from the apex of the semiconductor pyramid.

Referring first to FIG. 1, there is illustrated an AFM probe 10 thatincludes a semiconductor cantilever 12 having a semiconductor pyramid 14extending upward from a surface of the semiconductor cantilever 12 thatcan be employed in one embodiment of the present disclosure. As shown inFIG. 1, the semiconductor pyramid has an apex 15. The AFM probe 10 shownin FIG. 1 is present on a surface of semiconductor wafer (not shown). Insome embodiments (not shown), the AFM probe 10 is present in an array ofAFM probes 10. The array of AFM probes 10 would be similar to the arrayof AFM probes shown in FIG. 2 of U.S. Patent Application Publication No.2009/0308844 to Cohen et al., the entire content of which isincorporated herein by reference.

The term “semiconductor” is used throughout the present application todenote any semiconductor material such as, Si, Ge, SiGe, SiC, and SiGeC,in which semiconductor nanowires can be grown using an Al catalyzedprocess. The semiconductor material used in forming the AFM probe 10 mayalso comprise an organic semiconductor or a layered semiconductor suchas, for example, Si/SiGe, a silicon-on-insulator (SOI), aSiGe-on-insulator (SGOI) or a germanium-on-insulator (GOI). In someembodiments, the AFM probe 10 is composed of a Si-containingsemiconductor material, i.e., a semiconductor material that includessilicon. To reduce the cost of the AFM probe silicon (Si) is typicallyselected as the preferred semiconductor for making AFM probe 10.

The AFM probe 10 including cantilever 12 and pyramid 14 may be singlecrystalline, polycrystalline, or amorphous. In one embodiment, the AFMprobe 10 including the cantilever 12 and the pyramid 14 is singlecrystalline; the common fabrication method that is used to form pyramid14 is by anisotropic etching of single-crystal silicon. Thesemiconductor employed as the AFM probe 10 including the cantilever 12and the pyramid 14 may have any crystallographic orientation. Forexample, and when Si is employed as the semiconductor material, the Simay have a (111), (100) or (110) crystal orientation.

The AFM probe 10 shown in FIG. 1 can be formed utilizing techniques thatare well known in the art. For example, the AFM probe 10 can be formedby micromachining. In some embodiments in which the pyramid 14 iscomprised of Si, the pyramid 14 can be formed by anisotropic etching ofsilicon. For example, a masked etching of silicon with potassiumhydroxide (KOH) exhibits a very slow etching of Si (111) planes. As aresult, a pyramid whose sides are Si (111) planes is formed.

Referring now to FIG. 2, there shown the structure of FIG. 1 afterforming an oxide layer 16 on the AFM probe 10 including on the uppersurfaces of the semiconductor cantilever 12 and the semiconductorpyramid 14. In one embodiment, the oxide layer 16 that is formed iscomprised of a semiconductor oxide such as, for example, silicon oxide,or germanium oxide. In another embodiment, the oxide layer 16 iscomprised of a metal oxide. The oxide layer 16 may be comprised of asingle oxide material or, it may be comprised of a multilayered stack ofoxide materials such as, for example, a stack of a semiconductor oxideand a metal oxide including, for example, ZnO, NiO, HfO₂, ZrO₂, La₂O₃,Al₂O₃, TiO₂, SrTiO₃, LaAlO₃, Y₂O₃, HfO_(x)N_(y), ZrO_(x)N_(y),La₂O_(x)N_(y), Al₂O_(x)N_(y), TiO_(x)N_(y), SrTiO_(x)N_(y),LaAlO_(x)N_(y), and Y₂O_(x)N_(y). Each value of x is independentlyvaried from 0.5 to 3 and each value of y is independently varied from 0to 2. In embodiment of the present disclosure, the oxide layer 16 iscomprised of silicon oxide.

The thickness of the oxide layer 16 may vary depending on, for example,the technique used in forming the same as well as the type of oxidematerial that is employed in forming the oxide layer 16. Typically, theoxide layer 16 has a thickness that is greater than 3 nm, with athickness from 40 nm to 60 nm being more typical in some embodiments ofthe present disclosure.

The oxide layer 16 can be formed utilizing techniques that are wellknown to those skilled in the art. In one embodiment, the oxide layer 16is formed by a conventional thermal oxidation process. In anotherembodiment, the oxide layer 16 is formed by a deposition methodincluding, but not limited to, chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD), molecular beam deposition(MBE), pulsed laser deposition (PLD), liquid source misted chemicalvapor deposition (LSMCVD) and atomic layer deposition. Combinations ofthe above techniques can also be employed in forming the oxide layer 16.In one embodiment of the present disclosure, and when the oxide layer 16is silicon oxide, thermal oxidation (at a temperature from 700° C. to1100° C.) can be employed in forming the oxide layer. A low temperaturethermal oxidation exhibits a higher oxidation rate for Si (111) than forSi (100). This will result in a thicker oxide on Si (111) surfaces thanon Si (100). When the oxidation temperature is high the oxidation ratesis less dependent on the silicon surface orientation.

Referring now to FIG. 3, there is illustrated the structure of FIG. 2after forming a photoresist 18 on an upper surface of the oxide layer16. The photoresist 18 that can be employed in the present disclosureincludes a positive-tone photoresist material, a negative-tonephotoresist material or a hybrid photoresist that includes bothpositive-tone and negative-tone properties. The photoresist is formed onthe upper surface of the oxide layer 16 utilizing any conventionaldeposition process including, for example, spray coating or spin-oncoating. The thickness of the applied photoresist 18 has to be at leastlarger than the height of pyramid 14. As can be seen in FIG. 3, pyramid14 is submerged in photoresist 18. As can also be seen in FIG. 3 thephotoresist 18 planarizes the surface of the structure. Due toplanarization, the thickness of the photoresist 18 over the apex 15 isless than the thickness of the photoresist 18 over the cantilever 12.

Referring now to FIG. 4, there is illustrated the structure of FIG. 3after patterning the photoresist 18 into patterned photoresist 18′. Thepatterned photoresist 18′ includes an opening that exposes the oxidelayer 16 that is present on the apex 15 of the semiconductor pyramid 14.The patterning of the photoresist 18 into patterned photoresist 18′ canbe performed utilizing a controlled oxygen plasma ashing process. Thecontrolled oxygen plasma ashing process that can be used in the presentdisclosure to pattern the photoresist 18 removes the photoresistmaterial from the apex 15 of the semiconductor pyramid. The removal rateof the photoresist 18 by oxygen plasma ashing is uniform. Since the apex15 is the highest point in the structure, the apex 15 will be exposedfirst while the rest of the structure will remain submerged in thepatterned photoresist 18′. An optical end-point detection scheme can beused in some instances to accurately stop the resist etching when theapex 15 is exposed.

Reference is now made to FIG. 5 which depicts the structure of FIG. 4after removing the exposed oxide layer 16, not protected by thepatterned photoresist 18′, from the apex 15 of the semiconductor pyramid14. The removal of the exposed oxide layer 16, not protected by thepatterned photoresist 18′, exposes the apex 15 of the semiconductorpyramid 14. In FIG. 5, reference numeral 16′ denotes the patterned oxidelayer that is formed.

The removal of the exposed oxide layer is performed utilizing anyetching process that selectively removes oxide as compared to aphotoresist material, stopping on the underlying semiconductor surface.In one embodiment of the present disclosure, the removal of the exposedoxide layer 16, not protected by the patterned photoresist 18′, can beperformed utilizing an oxide etchant such as, for example, HF orbuffered HF. It is noted that when HF or buffered HF are employed, theexposed apex 15 of the semiconductor pyramid 14 is epitaxially clean. By“epitaxial clean” it is meant that the exposed apex 15 of thesemiconductor pyramid 14 is free of residual oxides or othercontaminates that may hinder the epitaxial growth of the semiconductornanowire from that surface.

Referring now to FIG. 6, there is illustrated the structure of FIG. 5after removing the patterned photoresist 18′ from atop the patternedoxide layer 16′. The removal of the patterned photoresist 18′ from thestructure includes any conventional resist stripping process. Forexample, acetone followed by isopropyl alcohol (IPA) can be used toremove the patterned photoresist from the structure. Alternatively,oxygen plasma can be employed to remove the patterned photoresist 18′.When oxygen plasma is employed, a diluted HF dip can be used to removeany thin oxide from the exposed apex 15 that may have formed duringoxygen plasma.

Next, and as shown in FIG. 7, a layer of aluminum 22 is formed on theremaining oxide layer, i.e., patterned oxide layer 16′, as well as onthe exposed apex 15 of the semiconductor pyramid 14. The layer ofaluminum 22 has a high affinity for reacting and forming a complex withoxygen that is present within the underlying patterned oxide layer 16′during a subsequent annealing process. It is noted that the layer ofaluminum 22 that is present on the exposed apex 15 of the semiconductorpyramid 14 will subsequently serve as a catalyst for nanowire growth.

The layer of aluminum 22 can be formed utilizing any deposition processincluding, for example, evaporation, chemical vapor deposition (CVD),ultra high vacuum chemical vapor deposition (UHVCD), plasma enhancedchemical vapor deposition (PECVD), atomic layer deposition (ALD),sputtering, and plating. Whichever technique is employed, the layer ofaluminum 22 should not be directly exposed to air or an oxygencontaining environment prior to nanowire growth. In one embodiment,evaporation is employed inside of the load lock chamber of an UHCVDsystem for forming the layer of aluminum 22 and the structure is notremoved from the UHV environment until after the nanowires are grown.

The thickness of the layer of aluminum 22 that is formed may varydepending on the technique used in forming the Al layer and the type ofpatterned oxide layer 16′ employed. The layer of aluminum 22 should bethin enough so that the chemical reaction with the underlying oxidelayer makes the entire Al layer over the patterned oxide layerunavailable to act as a seed for nanowire growth. Typically, thethickness of the layer of aluminum 22 is from 0.5 nm to 100 nm, with athickness from 2 nm to 10 nm being more typical.

Referring now to FIG. 8, there is shown the structure of FIG. 7 afterannealing. The annealing process employed causes the Al within layer 22to react with the oxygen within the patterned oxide layer 16′ forming apatterned compound oxide layer 24 that includes a combination of theelements within layers 16′ and 22. For example, when the patterned oxidelayer 16′ comprises silicon oxide, the annealing process provides apatterned compound oxide layer 24 that includes atoms of Al, Si andoxygen. The patterned compound oxide layer 24 is no longer active as aseed for nanowire growth.

It is observed that the above mentioned reaction that forms thepatterned compound oxide layer 24 occurs in areas in which the layer ofaluminum 22 is in direct contact with the patterned oxide layer 16′. Itis further observed that in those areas both layers 16′ and 22 can becompletely or partially consumed during this annealing process. In someembodiments, a portion of the patterned oxide layer 16′ which is inproximity to the semiconductor pyramid 14 is not completely consumed.

It is even yet further observed that in the area of the structure inwhich the layer of aluminum 22 is located directly on an exposed apex 15of the semiconductor pyramid 14, the aluminum balls up forming anon-oxidized semi-sphere or faceted crystal of aluminum. Thenon-oxidized semi-sphere or faceted crystal of aluminum can be used as anon-oxidized Al seed material 25 for nanowire growth.

The structure shown in FIG. 8 thus includes an area conducive fornanowire growth (e.g., the area including the non-oxidized Al seedmaterial 25), and areas that are not conducive for nanowire growth(e.g., areas including the patterned compound oxide layer 24).

The annealing may include a furnace anneal, a laser anneal, a microwaveanneal, or a rapid thermal anneal. In one embodiment, the annealincludes a furnace anneal in an UHVCVD chamber. Notwithstanding the typeof anneal employed, the anneal is typically performed at a temperaturethat is equal to, or greater than the eutectic temperature of an alloyof Al and the exposed apex 15 of the semiconductor pyramid 14. Thishelps in the Al material ball up. In some embodiments, the anneal isperformed at a temperature that is equal to, or greater than, 200° C. Inone embodiment in which Al on Si is employed, the anneal is typicallyperformed at a temperature above 577° C., with a temperature from 600°C. to 750° C. being more typical. In some embodiments in which theanneal is limited to the surface only, such as a laser anneal, highertemperatures can be employed. In one embodiment, the annealing processcan be conducted in an inert atmosphere including, for example, helium,argon, xenon and mixtures thereof. In yet another embodiment, a forminggas ambient can be employed. In yet another embodiment, an ultra highvacuum environment can be used. Irrespective of which type of inert orreducing atmosphere is used the level of oxygen must be such that the Alis not oxidized in the areas where the non-oxidized Al seed material 24is designed to form. The annealing process may employ a singletemperature, or various ramp and soak cycles can be employed.

Referring to FIG. 9, there is shown the structure of FIG. 8 aftergrowing a single semiconductor nanowire 26 from the non-oxidized Al seedmaterial 25. The growth of the single semiconductor nanowire 26, whichis assisted by the non-oxidized Al seed material 25, is typicallycarried out by chemical vapor deposition (CVD), ultra high vacuumchemical vapor deposition (UHCVD), or plasma enhanced chemical vapordeposition (PECVD) utilizing any of the well known semiconductornanowire precursors. The nanowire precursor can be in liquid form,gaseous form or in vaporous form. The growth temperature may varydepending on the type of semiconductor nanowire precursor employed.Typically, the semiconductor nanowire 26 is grown at a temperature from200° C. to 1400° C., with a growth temperature from 430° C. to 700° C.being more typical. The pressure employed in growing the semiconductornanowire 26 may vary depending on the growth temperature, the type ofprecursor employed as well as the type of reactor employed in formingthe nanowire. Typically, the pressure employed in forming thesemiconductor nanowire 26 is from 0.01 torr to 760 torr, with a pressurefrom 0.1 torr to 10 ton being more typical. Examples of semiconductornanowire precursors that can be employed in forming the semiconductornanowires 26 include any Si-containing precursor such, for example,silane (SiH₄) and chlorosilanes.

In one embodiment, and when silane (SiH₄) is used to grow siliconnanowires, the growth temperature is typically from 430° C. to 520° C.In another embodiment in which other precursors such as, for example,silicon tetrachloride (SiCl₄), are employed as the semiconductornanowire precursor the growth temperature is typically from 800° C. to1200° C. By adding chlorine to SiH₄, the growth temperature can beraised to above 600° C. The growth rate of the semiconductor nanowire 26depends on the growth temperature and the gas pressure in the growthchamber. For example, a typical CVD growth rate of silicon nanowiresgrown with SiH₄ diluted with H₂ (1:1) at a pressure of 0.5 ton and agrowth temperature of 490° C. is about 5.6 μm/hour.

Without wishing to be bound by any theory, the growth of semiconductornanowire 26 is believed to be best described by the vapor-liquid-solid(VLS) mechanism. See, for example, B. A. Wacaser et al. “PreferentialInterface Nucleation: An Expansion of the VLS Growth Mechanism forNanowires.” Advanced Materials 2009, 21 (2), 153-165. During theannealing step and or/when the growth is initiated, ametallic-semiconductor solid or liquid alloy is formed. With additionalsupply of semiconductor precursor from the gas phase, themetallic-semiconductor droplet becomes supersaturated with semiconductormaterial and the excess semiconductor is deposited at the solid-liquidinterface. As a result, the liquid droplet rises from the originalsubstrate surface to the tip of a growing nanowire crystal. Aftersemiconductor nanowire 26 growth, the metallic semiconductor liquidalloy will partially separate during cooling forming ametal-semiconductor solid alloy. As a result the liquid or solid alloyreverts back to material comprised primarily of the non-oxidized Al seedmaterial 25 after cooling.

The orientation of the semiconductor nanowire 26 can be the same as thatof the underlying apex 15 of the semiconductor pyramid 14 since it isepitaxially formed from that surface. The fabrication of nanocrystals bynanowire growth facilitates the formation of nanocrystals with nearidentical length, size, and orientation. Epitaxial nanowire growthtypically occurs in a (111) crystallographic growth direction, but canalso be achieved on other crystal orientations including, but notlimited to, (100), (110), (211) and (311).

The semiconductor nanowire 26 that is formed may comprise the same ordifferent semiconductor material as that of semiconductor pyramid 14. Inone embodiment, the semiconductor nanowire 26 is comprised of aSi-containing semiconductor material, with a silicon nanowire beingpreferred.

The length and aspect ratio of the semiconductor nanowire 26 that isformed can be controlled by selecting a certain temperature, durationand pressure during nanowire growth. The direction of the epitaxialgrowth of the semiconductor nanowire 26 is determined by the crystalorientation of the apex 15 of the semiconductor pyramid 14. For example,a vertical aligned, e.g. perpendicular, semiconductor nanowire 16 willgrow on a Si (111) surface, however, a tilted semiconductor nanowire 26with a certain angle will typically grow on a Si (100) surface.

The semiconductor nanowire 26 that is formed includes a p-dopedsemiconductor nanowire core 26A that is surrounded by an undopedsemiconductor shell 26B. The non-oxidized Al seed material 25 is presentat the tip of the p-doped semiconductor core 26A. The undopedsemiconductor shell is tapered because it grows at a constant rate onthe sides of the nanowire as it grows. The undoped semiconductor shell26B is thicker at the base of the nanowire because this part of thenanowire is grown first and thus the undoped semiconductor shell has alonger time to grow. The dopant from the seed material 25 does notdirectly affect the growth rate of undoped semiconductor shell 26B andthus it is not typically incorporated in the shell.

Referring to FIG. 10, there is shown the structure of FIG. 9 afterremoving, the non-oxidized Al seed material 25, the undopedsemiconductor shell 26B and the remaining oxide layer, i.e., compoundoxide 26 and/or remaining oxide layer 16′ not consumed during annealing,leaving a p-doped, i.e., aluminum doped, semiconductor nanowire 30(i.e., the p-doped semiconductor nanowire core 26A) on the apex 15 ofthe semiconductor pyramid 14.

The non-oxidized Al seed material 25 can be removed from the tip of thep-doped semiconductor nanowire core 26A by selective etching. Forexample, HF will selectively etch the non-oxidized Al seed material 25from the tip of the p-doped semiconductor nanowire core 26A. The undopedsemiconductor shell 26B can be removed utilizing an etching process thatselectively removes an undoped semiconductor material relative to adoped semiconductor. In one embodiment, tetramethylammonium hydroxide(TMAH) or potassium hydroxide (KOH) can be used to remove the undopedsemiconductor shell 26B. In some embodiments in which the exposed apexis a Si (111) surface, the exposed apex previously including the undopedshell portion typically does not etch in TMAH or KOH since these etchantare anisotropic and exhibit a very low etch rate for Si (111) surfacesrelative to other surface orientations. Additionally, it was found thatat low growth temperatures the undoped shell portion of thesemiconductor nanowire contains many crystal defects. These defectsfurther enhanced the etching rate of the shell portion of the nanowirewith respect to the semiconductor nanowire core.

The compound oxide layer 24, and optionally, any remaining portions ofthe patterned oxide layer 16′ not previously consumed during annealing,can be removed from the surface of the AFM probe 10 utilizing aconventional etching process that is capable of selectively removing thecompound oxide layer 24 and optionally any remaining patterned oxidelayer 16′ from the structure. In one embodiment, etching in HF can beused to selectively remove the compound oxide layer 24 and optionallyany remaining patterned oxide layer 16′ from the structure.

The surfaces (top and sidewalls) of the p-doped semiconductor nanowire30 and, if exposed, the underlying AFM probe 10 can be cleaned to removeany native oxide therefrom utilizing a stripping process that is wellknown to those skilled in the art.

The p-doped semiconductor nanowire 30 has a length, 1, as measured fromthe apex 15 of the semiconductor pyramid 14 to the tip of the p-dopedsemiconductor nanowire 30. The p-doped semiconductor nanowire 30 has anaspect ratio (length/diameter) of greater than 100:1. It is also notedthat no interface exists between the base of the p-doped semiconductornanowire 30 and the apex 15. The diameter of the p-doped semiconductornanowire 30 is from 20 nm to 200 nm; smaller diameters can be achievedby utilizing the thinning process illustrated in FIGS. 10 and 11, whichis described in greater detail herein below.

The p-doped semiconductor nanowire 30 that extends from the apex 15 ofthe semiconductor pyramid 14 is conductive and has a controllablediameter and a high aspect ratio, without significant tapering from thetip of the p-doped semiconductor nanowire to its base. Also, the p-dopedsemiconductor nanowire 30 that is provided by the present disclosure isstiff enough such that it can be inserted into a biological cell.Moreover, the method of the disclosure can be used to tailor thedimension and the direction of the p-doped semiconductor nanowire 30that is produced according to specific imaging needs. Furthermore, themethod of the present disclosure can also be employed for the massproduction of AFM tips which can be easily implemented into any standardSi AFM cantilever.

In some embodiments of the present disclosure, the p-doped semiconductornanowire 30 shown in FIG. 10 can be subjected to an optional thinningstep. When such a thinning step is employed the diameter of the thinnedp-doped semiconductor nanowire can be less than 20 nm. The optionalthinning step can be used to increase the resolution of the p-dopedsemiconductor nanowire 30 by decreasing the width of the semiconductornanowire. This aspect of the present disclosure which can be referred toas an oxidation sharpening technique is now described in greater detailby referring to FIGS. 11-12 and the following discussion. Referringfirst to FIG. 11, there is shown a bird's eye view of the pyramidalportion of the structure of FIG. 10 after performing an optional thermaloxidation step. The optional thermal oxidation step forms a surfaceoxide layer 50 on the p-doped semiconductor nanowire 30 as well as anyexposed semiconductor surfaces, such as, for example, the semiconductorpyramid 14 and the semiconductor cantilever 12.

The thickness of the surface oxide layer 50 that is formed may varydepending on the conditions of the thermal oxidation process employed.The surface oxide layer thickness will depend on the initial diameter ofthe p-doped semiconductor nanowire 30. As a rule of thumb, each 2.2 nmof oxide growth consumes 1 nm of silicon. As an example, assume that theinitial diameter of the p-doped semiconductor nanowire is 20 nm, andthinning to a diameter of 10 nm is desired. In such a case, one needs toconsume 5 nm from the nanowire radius which requires a growth of 11 nmof a surface oxide layer. The optional thermal oxidation step istypically performed at a temperature from 700° C. to 1100° C. Theoptional thermal oxidation step is performed in an oxygen-containingambient including, for example, O₂, H₂O, air, ozone and steam.Additional oxidation steps can be used to control thinning of thesemiconductor nanowire.

Referring now to FIG. 12, there is illustrated the structure of FIG. 11after removing the surface oxide layer 50 from the p-doped semiconductornanowire 30. The removal of the surface oxide layer 50 can be performedutilizing an etch that selectively removes oxide as compared to asemiconductor material. In one embodiment, Hf or buffered HF can beemployed to remove the surface oxide layer 50 from the p-dopedsemiconductor nanowire 30.

Reference is now made to FIGS. 13-16, which depict another embodiment ofthe present disclosure for defining the region at the apex of thesemiconductor pyramid in which a semiconductor nanowire will be growntherefrom. This embodiment relies on the different oxidation rates thatthe (111) and (100) planes have. In silicon, the (111) plane has thehighest oxidation rate while the (100) has the lowest oxidation rate. Itis noted that in this embodiment no photoresist is required in definingthe area for nanowire growth as was the case in the embodiment describedabove in regard to FIGS. 1-10.

Specifically, FIG. 13 shows an AFM probe 10′ that includes semiconductorcantilever 12 comprised of Si (100). The AFM probe 10′ also includes asemiconductor pyramid 14 having sidewalls 17 comprised of Si (111) andan apex 15 comprised of Si (100). The AFM probe 10′ is fabricated asdescribed above in regard to the AFM probe 10 shown in FIG. 1. FIG. 14shows the structure of FIG. 13 after forming an oxide layer 16 on theexposed semiconductor surface. The oxide layer 16 is formed by oxidationin this embodiment and because of the different oxidation rates that the(111) and (100) planes have, the oxide layer 16 has a differentthickness. For example, the oxide layer over the (100) planes is thinnerthan the oxide layer over the (111) planes. In FIG. 14, t₁ representsthe thinner portion of oxide layer 16 and t₂ represents the thickerportion of oxide layer 16. FIG. 15 shows the structure of FIG. 14 afterremoving the thinner portion t₁ of oxide layer leaving the thickerportion on the (111) Si sidewalls 17 of semiconductor pyramid 14; it isnoted that in this step the thicker portion t₂ is thinned to t₃ whereint₃≦t₂−t₁ (t₃ is equal or less than t₂−t₁). As is illustrated, the (100)surface at the apex 15 of the semiconductor pyramid 14 is exposed. Also,exposed is the (100) horizontal surface of the semiconductor cantilever12. The remaining thicker portion t₂ of oxide layer is labeled as 16′since it serves as a patterned oxide layer in the structure. The removalof the thinner portion t₁ of oxide layer 16 relative to the thickerportion t₂ of the oxide layer 16 is performed by etching in buffed HF ordiluted.

FIG. 16 illustrates the structure that is formed after semiconductornanowire 26 growth. In this embodiment, semiconductor nanowires willalso grow from the exposed horizontal surface of the semiconductorcantilever 12. The semiconductor nanowires that are grown from theexposed horizontal surface of the semiconductor cantilever are not shownin FIG. 16 so as not to obscure the growth of the semiconductor nanowireon the apex 15 of the semiconductor pyramid 14. The semiconductornanowires that will grow on the (100) horizontal surface of thesemiconductor cantilever will not interfere with the AFM imaging becausetheir height is much less than the combined height of the semiconductorpyramid 14 and the semiconductor nanowire 26 at the apex 15 of thesemiconductor pyramid 14. The semiconductor nanowire growth in thisembodiment includes forming a layer of Al on the structure of FIG. 15,annealing and epitaxial growing the semiconductor nanowire from anon-oxidized Al seed material 25. The formation of the layer of Al,annealing and epitaxial growth are the same as described above in FIGS.7, 8, and 9, respectively. Note that the steps shown in FIG. 10 can beperformed on the structure shown in FIG. 16 to provide a high resolutionAFM tip in accordance with the present disclosure. It is noted that inthis embodiment the semiconductor nanowire grown on a (100) plane is atan oblique angle)(54.7° to the (100) surface. This is a result of thenanowire principle axis being along the (111) direction. In someembodiments, the oxidation sharpening technique mentioned above can beperformed here for this embodiment as well.

Reference is now made to FIGS. 17-21, which represents a furtherembodiment of the present disclosure for defining the region at the apexof the semiconductor pyramid in which the semiconductor nanowire will begrown therefrom. This method does not depend on surface orientations butrather use the fact that many film depositions produce non-conformalfilms when the film is deposited over non planar surface (e.g. the facesof the pyramid). The deposited film is typically thinner on a verticalsurface and thicker on a horizontal surface. In this embodiment, the AFMprobe 10 illustrated in FIG. 1 can be employed.

Referring now to FIG. 17, there is shown the AFM probe 10 of FIG. 1after forming a non-conformal film 70 on the exposed surfaces of the AFMprobe. As mentioned above, the deposited non-conformal film 70 istypically thinner (t₁) on a vertical surface and thicker (t₂) on ahorizontal surface. The non-conformal film 70 is formed utilizing anynon-conformal deposition process such as, for example, evaporation andchemical vapor deposition (CVD). In one embodiment, the non-conformalfilm 70 may comprise a nitride such as silicon nitride. In anotherembodiment, the non-conformal film 70 may comprise a metal such as, forexample, Al. When Al is employed as the non-conformal film 70, theportion that is present atop the apex 15 of the semiconductor pyramid 14can be used to subsequently grow a semiconductor nanowire therefrom.

After deposition and as shown in FIG. 18, the non-conformal film 70 isetched with an isotropic etchant to expose the sidewalls 17 of thesemiconductor pyramid 14, but the thick portion of the non-conformalfilm 70 on horizontal surfaces is not fully etched. The unetchedportions of the non-conformal film 70 are now used as an oxidationblocking mask and are labeled as 70′ in FIG. 18. As is shown, oxidationblocking mask 70′ has a thickness t₃ which is smaller than originalthickness t₂ by more than the amount t₁ (t₃≦t₂−t₁).

The exposed sidewalls 17 of the semiconductor pyramid 14 are thermallyoxidized to form a patterned oxide layer 16′ thereon. The resultantstructure including the patterned oxide layer 16′ that is formed afterthermal oxidation is shown in FIG. 19. The thermal oxidation is the sameas described above in the first embodiment for forming oxide layer 16.

Reference is now made to FIG. 20, which shows the structure after thethermal oxidation blocking mask 70′ is removed from the horizontalsurface of the semiconductor cantilever 12 and optionally from the apex15 of the semiconductor pyramid 14. The embodiment shown in FIG. 20shows the removal of the thermal oxidation blocking mask 70′ from boththe horizontal surface of the cantilever and the apex of thesemiconductor pyramid. It is noted that in instances when Al is used asthe non-conformal film, the Al on the apex 15 of the semiconductorpyramid 14 may remain on the AFM probe and can be used as the catalystto grow the nanowire thereon. When the Al is to remain on the apex 15, ablock mask can be formed to protect the Al that is present on top theapex 15 of the semiconductor pyramid 14. The removal of the thermaloxidation blocking mask 70′ can be performed utilizing any etchingprocess that selectively removes the material of the thermal oxidationblocking mask 70′ relative to the patterned oxide layer 16′.

FIG. 21 illustrates the structure that is formed after semiconductornanowire 26 growth. The semiconductor nanowire growth in this embodimentincludes optionally forming a layer of Al on the structure of FIG. 20,annealing and epitaxial growing the semiconductor nanowire from anon-oxidized Al seed material 25. The optional formation of the layer ofAl, annealing and epitaxial growth are the same as described above inFIGS. 7, 8, and 9, respectively. Note that the steps shown in FIG. 10can be performed on the structure shown in FIG. 21 to provide a highresolution AFM tip in accordance with the present disclosure. In thisembodiment, semiconductor nanowires will also grow from the exposedhorizontal surface of the semiconductor cantilever 12. The semiconductornanowires that are grown from the exposed horizontal surface of thesemiconductor cantilever are not shown in FIG. 16 so as not to obscurethe growth of the semiconductor nanowire on the apex 15 of thesemiconductor pyramid 14. The semiconductor nanowires that will grow onthe (100) horizontal surface of the semiconductor cantilever will notinterfere with the AFM imaging because their height is much less thanthe combined height of the semiconductor pyramid 14 and thesemiconductor nanowire 26 at the apex 15 of the semiconductor pyramid14.

In either the embodiment shown in FIGS. 13-16 or the embodiment shown inFIGS. 17-21, the oxidation sharpening process described above can beemployed to further improve the resolution of the AFM tip.

While the present disclosure has been particularly shown and describedwith respect to various embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A method of fabricating a high resolution AFM tip comprising:providing an AFM probe including a semiconductor cantilever having asemiconductor pyramid extending upward from a surface of saidsemiconductor cantilever, said semiconductor pyramid having an apex;forming a patterned oxide layer on said AFM probe, wherein saidpatterned oxide layer has an opening that exposes the apex of saidsemiconductor pyramid; and forming a single p-doped semiconductornanowire on said exposed apex of said semiconductor pyramid utilizing anon-oxidized Al seed material as a catalyst for nanowire growth.
 2. Themethod of claim 1 wherein said forming the patterned oxide layercomprises forming an oxide layer on said AFM probe, forming a patternedphotoresist on said oxide layer, wherein said patterned photoresistincludes an opening to expose the oxide layer at the apex of thesemiconductor pyramid, removing the exposed oxide layer from said apexof the semiconductor pyramid, and removing the patterned photoresist. 3.The method of claim 2 wherein said forming the oxide layer comprisesthermal oxidation, said forming the patterned photoresist comprisesdeposition of a photoresist material and oxygen plasma ashing, and saidremoving the exposed oxide layer at the apex of the semiconductorpyramid includes etching in HF or buffered HF.
 4. The method of claim 1wherein said forming the patterned oxide layer comprises forming a oxidelayer having a thin portion located atop the apex of the semiconductorpyramid and a horizontal surface of the semiconductor pyramid and athick portion on sidewalls of the semiconductor pyramid, and removingthe thin portion from the apex of the semiconductor pyramid and from thehorizontal surface of the semiconductor pyramid.
 5. The method of claim4 wherein said apex of the said semiconductor pyramid and saidhorizontal surface of said semiconductor cantilever have Si (111) planesand said sidewalls of the semiconductor pyramid have Si (100) planes. 6.The method of claim 5 wherein said single p-doped semiconductor nanowireis perpendicularly orientated relative to the apex of the semiconductorpyramid.
 7. The method of claim 1 wherein said forming the patternedoxide layer includes forming a non-conformal film on said AFM probe,wherein said non-conformal film has a thick portion atop the apex of thesemiconductor pyramid and a horizontal surface of the semiconductorpyramid and a thin portion on sidewalls of the semiconductor pyramid,selectively removing the thin portion of the non-conformal film relativeto the thick portion of the non-conformal film to expose said sidewallsof said semiconductor pyramid, and performing a thermal oxidationprocess to form said patterned oxide layer on said exposed sidewalls ofsaid semiconductor pyramid.
 8. The method of claim 7 further comprisingremoving said thick portion of said non-conformal film said at leastsaid horizontal surface of the semiconductor pyramid and optionally fromsaid apex of the semiconductor pyramid after said thermal oxidation andprior to forming said p-doped semiconductor nanowire.
 9. The method ofclaim 8 wherein said non-conformal film comprises Al, and wherein saidthick portion of Al is not removed from atop the apex of thesemiconductor pyramid and serves as a catalyst for forming said singlep-doped semiconductor nanowire.
 10. The method of claim 9 wherein saidforming the p-doped semiconductor nanowire comprises annealing, andwherein said annealing forms said non-oxidized Al seed material used informing said single p-doped semiconductor nanowire.
 11. The method ofclaim 10 wherein said forming said single p-doped semiconductor nanowirefurther comprises selecting a semiconductor nanowire precursor,performing nanowire growth at a temperature from 200° C. to 2000° C.utilizing chemical vapor deposition, ultra high vacuum chemical vapordeposition or plasma enhanced chemical vapor deposition, and removing anundoped semiconductor shell from a p-doped semiconductor nanowire core.12. The method of claim 1 wherein forming the single p-dopedsemiconductor nanowire includes forming a layer of aluminum on saidpatterned oxide layer and on said exposed apex of said semiconductorpyramid, thermal annealing, wherein said thermal annealing forms acompound oxide layer in areas in which said layer of aluminum is indirect contact with said patterned oxide layer and the non-oxidized Alseed material on said apex of said semiconductor pyramid, growing asemiconductor nanowire from said non-oxidized Al seed material, whereinsaid semiconductor nanowire includes a p-doped semiconductor nanowirecore and an undoped semiconductor shell, and removing said undopedsemiconductor shell.
 13. The method of claim 12 wherein said growingsaid semiconductor nanowire includes selecting a semiconductor nanowireprecursor, and performing nanowire growth at a temperature of equal to,or greater than, 200° C. utilizing chemical vapor deposition, ultra highvacuum chemical vapor deposition or plasma enhanced chemical vapordeposition.
 14. The method of claim 1 further comprising removing saidnon-oxidized Al seed material from atop a tip of said single p-dopedsemiconductor nanowire.
 15. The method of claim 1 further comprisingsubjecting the p-doped semiconductor nanowire to an oxidation sharpeningprocess.
 16. The method of claim 15 wherein said oxidation sharpeningprocess includes thermal oxidation and removing a surface oxide layerfrom a thinned p-doped semiconductor nanowire.
 17. The method of claim 1wherein said single p-doped semiconductor nanowire has a diameter from20 nm to 200 nm, and an aspect ratio of greater than 100:1.
 18. A highresolution AFM tip comprising: an AFM probe including a semiconductorcantilever having a semiconductor pyramid extending upward from asurface of said semiconductor cantilever, said semiconductor pyramidhaving an apex; and a single Al-doped semiconductor nanowire on saidexposed apex of said semiconductor pyramid, wherein said single Al-dopedsemiconductor nanowire is epitaxial with respect to said apex of saidsemiconductor pyramid.
 19. The high resolution AFM tip of claim 18wherein said single Al-doped semiconductor nanowire has an aspect ratioof greater than 100:1 and a diameter from 20 nm to 200 nm.
 20. The highresolution AFM tip of claim 18 wherein said semiconductor cantilever,said semiconductor pyramid and said Al-doped semiconductor nanowirecomprise Si.
 21. The high resolution AFM tip of claim 18 wherein saidsingle Al-doped semiconductor nanowire is perpendicular to said apex ofsaid semiconductor pyramid.
 22. The high resolution AFM tip of claim 18wherein said single Al-doped semiconductor nanowire extends upward fromsaid apex of said semiconductor pyramid at an oblique angle.
 23. Thehigh resolution AFM tip of claim 18 wherein said single Al-dopedsemiconductor nanowire has a diameter of less than 20 nm.